A. Field of the Invention
The present invention relates generally to the field of liquid filtration, and in particular to a precoat filter with enhanced efficacy and minimum loss of throughput.
B. Description of the Related Art
Water clarification, particle removal and wastewater treatment are presently performed by a number of different types of systems. These include chemical coagulation, chemical precipitation, flocculation, sedimentation, sand filtration (see, e.g., U.S. Pat. No. 3,680,699), chlorination, packed-bed ion exchange, packed-bed granular activated carbon, fluidized-bed granular activated carbon, alkaline neutralization, alkaline supplementation using soda ash, oxidation, aeration, single-stage precoat diatomaceous earth (DE) filtration, fabric screen filtration, metal screen filtration, membrane microfiltration, membrane ultrafiltration, reverse osmosis, electrodialysis, flotation and various biological processes.
In general, these systems suffer from high production cost, high operational cost, complexity, and/or unreliability. Furthermore, demanding federal and state standards for potable water prevent the use of many conventional filtration methods in drinking-water applications. Even non-potable wastewater treatment guidelines have become increasingly stringent, placing strains on the limits of current technology to deliver high filtration efficacy with acceptable throughput.
Mechanical methods of filtration typically operate by physical exclusion. The contaminated fluid is passed through a porous medium, which retains particles larger than the size of the pores but permits passage of the fluid effluent (containing particles smaller than the pores) therethrough. Decreasing the size of the pores improves the quality of the effluent, but at the expense of throughput; greater hydraulic capacity is necessary to force the same quantity of fluid per unit time through smaller pores. The pressure differential between the interior and exterior of the septum at a given hydraulic capacity, which represents the loss in hydraulic efficiency due to resistance of the filtration element, is known as head loss or pressure loss.
As used herein, "filter performance" denotes an evaluation of a given filter based on the size of the smallest particle that can be removed at a given throughput rate per unit of filter area using a given input pressure; "filtration efficacy" refers to the absolute size of the smallest particle that can be removed by a particular system at any operational pressure and throughput rate per unit of filter area. Because some distribution of filtrate particles is inevitable, a more common measure of efficacy is turbidity. This quantity reflects the cloudiness of the filtrate, and is typically expressed in nephelometric turbidity units (NTU) of visual clarity. Filtration to a turbidity level of 10-15 NTU is called microfiltration; achieving a level of no more than 1 NTU is called ultrafiltration.
It is well-known that depositing a bed of porous granular material along a porous solid support, or septum, will yield greater filter performance than that of the septum alone. The bed in effect adds a third dimension of filtration to the two-dimensional septum; however, because the granular material is itself porous in three dimensions, high degrees of filtration efficacy may be achieved with minimal loss of fluid throughput. In these systems, the principal function of the septum is retention of the granular material rather than actual filtration.
Prior to introduction of the contaminated fluid, the septum must be exposed to a slurry containing the granular material. This is known as the "precoat" step, and the granular portion of the slurry is referred to as the precoat material. This material should consist of particles having a distribution of sizes for optimum filtration. However, the septum pores should remain somewhat larger than the smallest precoat particles to reduce head loss. Retention of the precoat particles along a support having pores larger than some of these particles is typically accomplished by recirculating the precoat material through the support until the effluent appears relatively free of precoat material, indicating that the smaller particles have lodged behind the larger ones. This process consumes time and energy.
Because of its high porosity, DE is commonly used in precoat filtration systems. DE consists of extremely tiny fossil-like skeletons of microscopic aquatic plants called diatoms. Each skeleton is a highly porous framework of nearly pure silica, and may range from 0.5 to 100 microns in diameter. DE is available in different size distributions from a number of manufacturers. Other precoat media include adsorbents (e.g. activated carbons and synthetic polymeric adsorbents), cation exchangers and anion exchangers for dissolved impurities, and activated alumina.
Once filtration begins, solid contaminants that are removed from the fluid remain within the precoat layer. These deposits form a layer surrounding the precoat layer as filtration continues. Because the particles comprising this layer are not porous, the deposited layer resists passage of the fluid to a greater extent than the precoat layer, resulting in deterioration of filter performance over time. This deterioration can be forestalled by adding precoat material during the filtration process, a technique known as "body feeding." The body feed acts to distribute the solids embedded within the original precoat, thereby deterring buildup of an impervious layer. Of course, the fluid can be forced through the septum at a higher pressure to overcome the buildup of solids, but the increased head loss would require delivery of significantly more hydraulic energy to preserve throughput. As a practical matter, increasing input pressure requires expensive variable-speed pumping equipment.
Regardless of the ameliorative measures taken, a point will inevitably be reached at which fluid will be unable to pass through the septum with acceptable throughput characteristics relative to hydraulic energy input, necessitating removal of the solid contaminants and precoat layer from the septum. This "backwashing" step consumes time and energy, interrupts the filtration cycle, and requires a subsequent precoating procedure.